ANALYSIS AND DESIGN OF A SIMPLE SURGE TANK
Introduction: When the valve in a hydroelectric power plant is suddenly completely closed, because of its small inertia the water in the penstock stops almost at once.The water in the pipeline, with large inertia retards slowly. The difference in flows between pipeline and penstock causes a rise in the water level in the surge tank. The water level rises above the static level of the reservoir water, producing a counter-pressure so that water in the pipeline flows towards the reservoir and the level of water in the surge tank drops.
In the absence of damping, osciJIation would continue indefinitely with the same amplitude. The extent of damping is governed by roughness condition, restricted orifice, and so on. The flow into the surge tank and water level in the tank at any time during the osculation depends on the dimension of the pipeline and tank and on the type of valve movement. The main functions of a surge tank are:
1. It reduces the amplitude of pressure fluctuations by reflecting the incoming pressure waves;
2. It improves the regulation characteristics of a hydraulic turbine.
MAIN CONSIDERATIONS IN THE DESIGNOF A SURGE TANK: in order to accomplish its mission most effectively, the surge tank dimensions and location are based on the following considerations:
I. The surge tank should be located as close to the power or pumping plant as possible;
2. The surge tank should be of sufficient height to prevent overflow for all conditions of operation;
3. The bottom of surge tank should be low enough that during its operation the tank is drained out and admit air into the turbine penstock or pumping discharge line; and
ANALYSIS OF SURGES IN SIMPLESURGE TANK: In a simple surge tank, there is very little head loss between the surge tank and the pipeline, also the reservoir is considered so large that its level remains constant.
Derivation of Governing Equation: To simplify the derivation of dynamic and continuity equations that describe the oscillations of the water level in the tank, it has been assumed that
(i) The conduit walls are rigid;
(ii) The water is incompressible; and
(iii) The effect of entrance loss in comparison with the friction loss has been neglected. The equation of motion is written as
Integration (l.b) with respect to x between the limits x=O,x=L (see Figure I) and simplifying, one gets
From continuity condition between tank and pipe,it can be shown that
That with use of 3 in 2 the following equation can be found:
With initial condition at t=O